Polytelluride square planar chain induced anharmonicity results in ultralow thermal conductivity and high thermoelectric efficiency in Al2Te5 monolayers

Two-dimensional (2D) metal chalcogenides provide rich ground for the development of nanoscale thermoelectrics, although achieving optimal thermoelectric efficiency has yet to be a challenge. Here, we leverage the unique chemistry of tellurium (Te), renowned for its hypervalent bonding and catenation abilities, to tackle this challenge as manifested in Al₂Te₃ and Al₂Te₅ monolayers. While the former forms a straightforward covalent Al–Te network, the latter adopts a more intricate bonding mechanism, enabled by eccentric features of Te chemistry, to maintain charge balance. In Al₂Te₅, a square planar chain (SPC) known as polytelluride [Te₃]^2- is neutralized by covalently bonded [Al₂Te₂]²⁺ framework. The hypervalent nature of Te results in bizarre Born effective charges of 7 and -4 for adjacent Te atoms within the square planar chain, the feature that induces significant anharmonicity, and leads to a glass-limit of lattice thermal conductivity (κ_L) in Al₂Te₅ monolayers. Enhanced anharmonic lattice vibrations and the accordion pattern bestow glass-like, strongly anisotropic thermal conductivity to the Al₂Te₅ monolayer. The calculated κ_L values of 1.8 and 0.5 Wm^-1K^-1 along the a- and b-axes at 600 K are one order of magnitude lower than those of Al₂Te₃, and even lower than monolayers that contain heavy cations like Bi₂Te₃. Moreover, the tellurium chain dominates the valence band maximum and conduction band minimum of Al₂Te₅, leading to a high valley degeneracy of 10, and thus a high power factor and figure of merit (zT). Using rigorous first-principles calculations of electron relaxation time, the estimated hole-doped and electron-doped zT of, respectively, 1.5 and 0.5 at 600 K is achieved for Al₂Te₅. The pioneering zT of Al₂Te₅ compared to that of Al₂Te₃ is rooted merely in its amorphous-like lattice thermal transport and its polytelluride chain. These findings underscore the importance of aluminum telluride and polymeric-based inorganic compounds as practical and cost-effective thermoelectric materials, pending further experimental validation.